This application is related to, and claims priority from, Japanese Patent Application No. 2001-173832, filed on Jun. 8, 2001, the entirety of which is incorporated by reference herein.
1. Field of the Invention
The present invention relates generally to motorcycle frame assemblies and, more particularly, to an improved motorcycle frame assembly that achieves a desirable balance between rigidity in a lateral direction and rigidity in a vertical direction.
2. Description of the Related Art
As is well known, motorcycles generally include a frame assembly that is made up of a plurality of individual tubes welded together. The typical frame construction includes a head pipe at a front end of the frame, which desirably supports the front wheel via a steerable front fork assembly. One or more main tubes typically extend generally rearwardly from the head pipe. The main tubes may bend downward over a rear portion of an engine that is positioned generally below the main tubes. In addition, one or more down tubes are typically affixed at their forward ends to the head pipe at a location lower than the main tubes. The down tubes extend downwardly and rearwardly from the head pipe. The rear ends of the main tube and the down tubes generally are joined, frequently by a pair of seat rails and bracket assemblies. One of the bracket assemblies typically provides support for the rear wheel suspension swing arm assembly. Although the arrangement of the individual frame members may vary, the above-described construction is generally the type utilized for motorcycles in general and off-road motorcycles in particular.
In off-road motorcycles, it is often desirable for the motorcycle frame to possess differing levels of rigidity in the lateral direction and the vertical direction. This is due, at least in part, to an off-road motorcycle being subjected to forces occurring from both cornering and jumping.
In the case of lateral loads, rigidity is desired to improve stability during cornering at high speeds or over rough terrain. In the case of the vertical loads, the motorcycle may land from considerable heights, which causes large shock loads to be applied to the frame. If the frame is too rigid, the shock loads are largely transferred to the rider. If the frame is not rigid enough, the frame may elastically deform or break.
U.S. Pat. No. 5,845,728, issued to Hiroyuki Itoh et al. on Dec. 8, 1998, describes several embodiments intended to increase the transverse rigidity of a motorcycle frame while not overly increasing the vertical rigidity of the motorcycle frame. The disclosed embodiments feature main tubes that feature flat upper and lower faces. Thus, in some embodiments, the main tube is rectangular with a larger lateral dimension than a vertical dimension. In the arrangement shown in FIG. 12 of the ""728 patent, the main pipe is disclosed as having an elliptical cross-section with flat upper and lower surfaces that facilitate attaching components of the motorcycle to the frame and rounded side walls that supposedly enable a characteristic design. It is clear from this description and the cross-section shown in FIG. 13 that the main pipe is oval and not elliptical in shape.
The oval shape and the rectangular shape of the main pipe in the ""728 patent increase the transverse rigidity of the frame assembly while not overly increasing the vertical rigidity of the frame assembly. The disclosed shapes, however, each feature a stress-rising construction. In the case of the rectangle, the corner at which two perpendicular faces meet defines a distinct stress-riser. In the case of the oval, the rather abrupt change in curvature defined between the flat upper and lower surfaces and the curved side faces also defines a stress-riser. In order to adequately compensate for the stress-risers in an off-road motorcycle frame, the main pipe diameter would need to be greatly increased. The increase in diameter, however, approximates a circular design, which suffers from being too rigid vertically when adequately rigid laterally.
Accordingly, preferred embodiments of the present motorcycle frame assembly include a main tube that, in section, is non-circular but continuously curved with a lateral dimension that is larger than the vertical dimension. Such an arrangement advantageously provides lateral rigidity while permitting at least a small amount of compliance in the vertical direction. In addition, desirably the main tube curves downwardly from the front end toward the center of the main tube and curves upwardly from the center toward the rearward end of the main tube. With such an arrangement, the ability of the main tube to experience elastic deformation in response to vertical loads is increased in comparison to main tube that is linear. As a result, the motorcycle frame exhibits exceptional lateral rigidity to improve handling, while providing an advantageous amount of vertical compliance to improve the comfort of the rider.
An aspect of the present invention involves an off-road motorcycle comprising a frame assembly. The frame assembly includes a head pipe, a main tube and a down tube. The main tube and the down tube extend in a generally rearward direction from the head pipe and the main tube is positioned above the down tube. A front suspension assembly is rotatably supported by the head pipe and rotatably supports a front wheel. A rear suspension assembly is pivotably supported by the frame assembly and rotatably supports a rear wheel. An internal combustion engine is supported by the frame assembly and is configured to drive the rear wheel. The main tube defines a first cross-sectional dimension in a first direction substantially aligned with a vertical, center plane of the motorcycle and a second cross-sectional dimension in a second direction substantially normal to the first direction. The second dimension is greater than the first dimension. The main tube is nonlinear along its length and defines a center axis extending from a forward end of the main tube to a rearward end of the main tube. A center portion of the center axis is spaced below a straight line extending between the forward end and the rearward end of the axis.
Another aspect of the present invention involves a motorcycle frame assembly comprising a head pipe, a main tube and a down tube. The main tube and the down tube extend in a generally rearward direction from the head pipe. The main tube is positioned above the down tube. The main tube defines a first cross-sectional dimension in a longitudinal vertical plane of the motorcycle and a second cross-sectional dimension in a second direction substantially normal to the first direction. The second dimension is greater than the first dimension and the main tube has a noncircular continuously curved cross-section along at least a portion of the main tube.
A further aspect of the present invention involves a motorcycle frame assembly comprising a head pipe, a main tube, a down tube and a seat rail. The main tube and the down tube extend in a generally rearward direction from the head pipe. The main tube is positioned above the down tube and the seat rail extends between rearward ends of the main tube and down tube. The main tube, down tube and seat rail are arranged in a generally triangular configuration and define a generally triangular internal space therebetween. The main tube defines a first cross-sectional dimension in a first direction. The first direction is substantially aligned with a vertical, center plane of the motorcycle. A second cross-sectional dimension is defined by the main tube in a second direction substantially normal to the first direction. The second dimension is greater than the first dimension. The main tube is arcuate along its length such that the main tube defines a convex outer surface portion opposing a concave outer surface portion. The main tube is arranged such that the convex outer surface generally faces the internal space.